29 Tectonic Significance of Buried Terranes of the Atlantic and Gulf Coastal Plains

Monday, 15 March 2010
8:00 AM - 12:05 PM
Liberty B

BASEMENT TERRANES OF THE CHESAPEAKE BAY REGION – EVIDENCE FROM BOREHOLE SAMPLES AND GEOPHYSICAL SURVEYS

Paper number: 29-4
Program: Theme Sessions
Time: 8:45 AM – 9:05 AM

Authors

Abstract

Studies of borehole samples and potential field data from the Chesapeake Bay impact structure (CBIS) and surrounding areas provide new insights into the geology of coastal-plain basement from the Chesapeake Bay suture zone (CSZ) east to the Delmarva Peninsula. The CSZ, including the Sussex terrane, coincides with the Sussex-Leonardtown (Salisbury) anomaly (gravity high, magnetic high) and separates NW-trending anomalies to the east from NE-trending anomalies typical of Appalachian Piedmont terranes to the west. However, the lithostratigraphy, age, and kinematics of the CSZ are poorly constrained. Recent USGS drill cores from the York-James and Middle Peninsulas west of Chesapeake Bay recovered Neoproterozoic granites (in situ) and lower greenschist-facies metavolcanic rocks (reworked CBIS ejecta) similar to those of exposed peri-Gondwanan terranes. In contrast, cores from the southern Delmarva Peninsula (Eyreville site) recovered a lower section of multiply-deformed, middle to upper amphibolite-facies graphitic schists with calc-silicate gneiss, mylonite, and pegmatitic granite. Whether or not these rocks represent a northern extension of the Hatteras terrane, contrasting metamorphic grades suggest that the CBIS straddles a crustal boundary between lower greenschist-facies rocks on the west and amphibolite-facies rocks on the east. The Eyreville cores also recovered a slab of Neoproterozoic gneissic granite intruded by Permian (late Alleghanian) granite similar to that of the Portsmouth pluton (also Permian), a block of orthoamphibolite, and clasts that include greenschist-facies metamorphic rocks, undated granitoids, and subgreenschist-facies gray and black shales. The latter are too thermally mature to be derived from coastal-plain sediments or Mesozoic rift basins and are interpreted to represent a previously unknown sequence of uncertain (Paleozoic?) age and tectonic affinity. Crystalline basement is cut by NE- and N-trending extensional faults related to Triassic/Early Jurassic rift basin formation; the northeast end of the Taylorsville basin is bounded by the CSZ, suggesting tectonic inheritance. Cretaceous/Cenozoic passive-margin tectonism deformed the basement and overprinted the rift basins, producing the Salisbury embayment and the James River structural zone.